Add vignette example

inst/rmd/DelayedArray.Rmd

@@ -9,6 +9,8 @@ library(Rarr)
 interface to Zarr array, and use delayed operations and block processing to 
 efficiently work with large on-disk data.
 
+### Working with an existing Zarr array
+
 To demonstrate using the DelayedArray framework, we'll pick the example file containing
 32-bit integers arranged in the "column first" ordering.
 
@@ -38,3 +40,11 @@ dim(zarr_array)
 chunk_dim(zarr_array)
 ```
 
+### Realizing an in-memory array to Zarr
+
+```{r}
+X <- matrix(rnorm(1000), ncol=10)
+zarr_path <- tempfile(fileext = ".zarr")
+zarr_X <- writeZarrArray(X, zarr_array_path = zarr_path, chunk_dim = c(10,10))
+zarr_X
+```